1. Field of the Invention
This invention relates generally to computer cables, and more particularly to methods and apparatus for efficiently transmitting high-speed serial data between a computer system and a peripheral.
2. Description of the Related Art
The transmission of information between electronic devices has been a concern since the development of the first computers. Speeds of transmission that might have once been considered fast some ten or even five years ago, would be considered a bottleneck by today's standards.
Historically, computers typically communicated by serial transmission. That is, one computer would send information to another computer a single bit at a time. Due to the relative slow clock speeds of the integrated circuits, the speed of serial data transmission is intrinsically limited.
In contrast, parallel data transmission allows a computer to send more than one bit of information at a single clock cycle. Parallel transmission requires a cable typically having many electric wires. Typically, a parallel cable has enough wires to transmit at least an eight bit word of information, thus requiring at least eight electric wires. For example, common interfaces on personal computers are parallel port and Small Computer Systems Interface (SCSI) ports. These and other parallel port configurations are designed to transmit from 8 to 16 or more bits of information per clock cycle. In order to transmit the information from one computer to another, a parallel port cable has over 20 conductors. Obviously, a cable having over 20 conductors becomes more cumbersome. In addition, the distance over which parallel cables are effective is quite limited due to synchronization factors between related bits in the various wires of the cable.
With recent advances in the speeds of the integrated circuits used to send and receive digital information, and because of the greater transmission distances possible with serial cables, there has be a return towards serial data transmission. Currently several standards for serial data transmission exist, such as ethernet, Localtalk and RS-422. Typically these types of serial transmission systems can transmit information up to about 10 megabits per second. More recently, the computer industry has been driven towards a higher speed serial data transmission standard, especially for communication between peripheral devices.
One drawback to higher speed serial data transmission is the requirement to keep electromagnetic emissions of the cables to a minimum. Under Federal Communications Commission (FCC) regulations, cables are limited as to the amount of electromagnetic radiation they can emit at certain frequencies. As seen in Table 1, the FCC's class B limits prohibit emissions by cables over 37 dB micro-volts per meter for frequencies at or below 540 MHz.
TABLE 1 ______________________________________ Frequency (MHz) Class B Limit (dB.mu.V/m) ______________________________________ 30 &lt; f .ltoreq. 230 30 230 &lt; f &lt; 1000 37 ______________________________________
Recently, the electronics industry began development and standardization of a high-speed serial data transmission architecture. In 1995 the Institute of Electrical and Electronic Engineers (EEE) approved the standard for the new high-speed serial data transmission architecture. The standard is known as 1394-1995 IEEE Standard for a High Performance Serial Bus, incorporated herein by reference, one implementation of the standard commonly referred to as FireWire.RTM. which can be obtained from the Institute of Electrical and Electronics Engineers, Inc., 345 East 47th Street, New York, N.Y. 10017. The purpose of the 1394 standard is to provide a high-speed low cost serial bus for use as a peripheral bus or a parallel back-plane bus.
One of the advantages of the 1394 standard is the ability to transmit data over a cable medium at variable speeds, including very high speeds. Transceiver chip sets for the 1394 standard are now running at speeds of up to 400 Mbps, and many companies anticipate reaching speeds of up to 1 Gbps. However, the basic clocking frequency of the 1394 standard is 24.576 MHz, and data is transmitted in multiples of 24.576 MHz.
Referring initially to FIG. 1a, a typical 1394 cable is described. A personal computer 10 contains a 1394 peripheral card 12. Transceiver chips (not shown) in the 1394 peripheral card drives a six pin input/output port. A peripheral device 16 is typically connected to the computer 10 by a 1394 cable assembly or "cable" 20, by way of its six pin input/output port. The peripheral 16 can be almost any type of electronic device, such as a video cassette recorder, stereo system, home theater system or a camcorder, as long as it has the appropriate 1394 standard equipment to support communication with the computer 10. Additionally, peripheral devices can communicate with each other via the 1394 protocols.
Referring to FIG. 1b, the 1394 cable 20 has two connectors 21 and 22, and a cable portion 23. As seen in FIG. 1c, each connector 21 and 22 has six pins. The designation of the pins are shown in Table 2.
TABLE 2 ______________________________________ Pin Signal Name Comment ______________________________________ 1 VP Cable Power 2 VG Cable Ground 3 TPB* Strobe on receive, data on transmit 4 TPB (differential pair) 5 TPA* Data on receive, strobe on transmit 6 TPA (differential pair) ______________________________________
Each connector 21 and 22 has a power pin 21(1) and 22(1), respectively, a ground pin 21(2) and 22(2), respectively, and two pairs of signal pins 21(3)-(6), and 22(3)-(6), respectively. The power pins 21(1) and 22(1) are coupled together by a conductor (i.e., an insulated wire) 31. The ground pins 21(2) and 22(2) are coupled together by conductor 32. Each pair of single pins correspond to a single twisted wire pair of conductors. Pins 3 and 4 of the connectors 21(3)-(4), and 22(3)-(4) constitute twisted wire pair B of the respective connectors 21 TPB and 22 TPB. Pins 5 and 6 of the connectors 21(5)-(6) and 22(5)-(6) constitute twisted wire pair A of the respective connectors 21 TPA and 22 TPA. Each twisted pair carries a single differential signal.
The nature of the 1394 protocol allows devices to be chained together. The 1394 protocol requires two-way communication between devices in a branch-leaf chain. Cable 20 used to connect the devices contain two twisted wire pairs to allow two-way communication. One twisted pair carries the data signal and the other twisted pair carries a strobe signal. The designation of which twisted pair is to carry the data or the strobe is dependent upon which device is sending or receiving the data. For example, using the configuration depicted in FIGS. 1a-1c, when computer 10 is transmitting it sends data out on its twisted pair B and a strobe on its twisted pair A. If cable 20, depicted in FIGS. 1b-c, is used to connect computer 10 and peripheral 16, computer 10 would be sending data on twisted pair 21 TPB, and a strobe on twisted pair 21 TPA. When computer 10 is receiving information, it receives data on twisted pair A (21 TPA) and the strobe on twisted pair B (21 TPB). The same transmission and reception scheme is true for peripheral 16, but in relation to the twisted pairs of its connector 22.
If computer 10 is transmitting data on the computer's twisted pair B (22 TPB), the peripheral should be receiving data on its twisted pair A (22 TPA). Thus, the twisted wire pairs are crossed in the cable. Again, the wiring diagram of the cable is depicted in FIG. 1c. Using the example of a 1394 standard configuration depicted in FIG. 1a, pins 3 and 4 of the first connector 21(3) and 21(4) represent twisted wire pair B of the computer (22 TPB), or for the purposes of this example, the data output of computer 10. Pins 3 and 4 21(3) and 21(4) are connected to the twisted pair wires 33 and 34, respectively. The twisted pair wires 33 and 34 are then connected to pins 5 and 6 of the second connector 22(5) and 22(6). Pins 5 and 6 of the second connector 22 represent twisted wire pair A (22 TPA) with respect to the peripheral 16, or the data receive of the peripheral 16.
Similarly, the strobe output of computer 10 is represented by pins 5 and 6 of the first connector 21(5) and 21(6), representing the computer's twisted wire pair A (21 TPA), is carried on the twisted wires 35 and 36, respectively. And, the twisted wires 35 and 36 are connected to pins 3 and 4 of the peripheral 16 to become its strobe input, or twisted wire B (22 TPB). As can be seen, when the peripheral 16 is transmitting data, the computer receives the data and the strobe on its appropriate twisted wire pairs.
Importantly, cable portion 23 usually has inner shields 24a and 24b surrounding each twisted wire pairs 33-34 and 35-36. The inner shields 24a and 24b are typically electrically coupled together by a galvanic connection 38 and to the ground wire 32. The cable portion 23 also has an outer shield 24c. Outer shield 24c is typically coupled to the housings of the two connectors by a low impedance coupling 39. For standard 6-to-6 connectors, this scheme of shielding has been adequate to meet FCC standards at data transmission rates of 200 Mbps, mainly due to the fact that the cable 23 carries a ground wire to which it can ground its inner shields 24a and 24b, and the outer shield 24c is independently grounded through the connector housings.
Cable 20 carries a power 31 and a ground wire 32 because the 1394 standard allows for devices to draw power from other devices. This is a useful feature of the 1394 standard, however not all devices require power from cable 20. Thus, some companies have utilized cables that do not carry power. For example, the Sony Corporation has utilized 4-to-4 pin connectors and cables for its camcorder products, as seen in FIG. 2a. The 4-to-4 cable 60 couples two camcorders 50 and 52. Each camcorder 50 and 52 has its own power supply, thus, no longer requiring the power and ground wires. By reducing the number of pins and conductors required in the cable, the size of the connectors have been reduced to about a third of the size of the standard 6-to-6 pin cable connector. For applications such as camcorders, the reduction in size of the connectors is a definite advantage.
A potential problem with the 4-to-4 pin cable is the electromagnetic performance of the cable. Referring to FIG. 2b, a 4-to-4 pin cable 60 is depicted. Cable 60 has two connectors 61 and 62, and a cable portion 63. Referring to FIG. 2c, the connectors 61 and 62 have a total of four pins 61(1)-(4) and 62(1)-(4), respectively. Corresponding to the configuration of the signal pins of the standard 6-to-6 cable, pins 1 and 2 61(1)-(2) of the first connector 61 are coupled to pins 3 and 4 62(3)-(4) of the second connector 62 by conductors 71 and 72, respectively. Again, following the 1394 protocol, the twisted pair A of one connector is connected to the twisted pair B of the other connector. Pins 3 and 4 61(3)-(4) of the first connector 61 are coupled to pins 1 and 2 62(1)-(2) of the second connector 62. Thus, pins 1 and 2 of the connectors 61 and 62 represent twisted pair B (61 TPB and 62 TPB), and pins 3 and 4 represent twisted pair A (61 TPA and 62 TPA), of the respective connectors 61 and 62. The cable portion 63 may or may not have inner shields 64a and 64b, but typically has an outer shield 64c.
Since there are no ground or power pins on connectors 61 and 62, the outer and inner shields of the 64a-c are not coupled to a ground, i.e., they are "floating". Because the shields 64a-c are not grounded, the cable 60 can exhibit high levels of electromagnetic radiation when carrying high frequency data. That is, when data is being transmitted through the cable at high-speeds, it is possible that the cable can emit electromagnetic radiation in excess of the FCC limits.
Another problem with the four pin configuration for 1394 standard applications is the inability to communicate with devices with the standard six pin configuration. Referring to FIG. 3a, a user may wish to connect a computer 10 having a six pin connection to a camcorder 52 having a four pin connection. One solution has been to create a 6-to-4 cable 80 by mating a six pin connector with a four pin connector using a four wire cable. The 6-to-4 cable configuration is referred to as a AV 1394 cable.
Referring to FIGS. 3a and 3b, a typical 6-to-4 pin cable 80 of the AV 1394 type cable is depicted. The cable 80 has a standard 6 pin connector 81 and a four pin connector 82. The signal pins 81(3)-(6) and 82(1)-(4) of the connectors are connected in accordance with Table 3, below, through conductors 93-96, respectively.
TABLE 3 ______________________________________ Pin # of 6 Pin Pin # of 4 Pin Connector Description Connector Description ______________________________________ 1 VP -- 2 VG -- 3 TPB* 3 TPA* 4 TPB 4 TPA 5 TPA* 1 TPB* 6 TPA 2 TPB ______________________________________
The cable 80 may or may not have inner shields 84a and 84b disposed around the twisted pair wires. Typically the cable 80 will have an outer shield 84c. Outer shield 84c is typically coupled to the ground pin 81(2) of the six pin connector 81 by a conductor 92.
Again, the 6-to-4 cable 80 can fail to provide adequate electromagnetic shielding during the transmission of data. While attempting to solve the problem of connecting a six and a four pin connector, the 6-to-4 pin cable 80 was not designed to reduce electromagnetic emissions. The connection of outer shield 84c to ground pin 81(2) does allow for a return path for direct currents, but is typically inadequate to shunt alternating currents generated within the shields 84a-c during the transmission of high-speed serial data.
Table 4 shows the electromagnetic output of a 6-to-4 cable operating at various high clock rates, in multiples of the basic clocking rate of 24.576 MHz, using a peak detector.
TABLE 4 ______________________________________ Electromag Emis- netic FCC Class sion - Table Antenna Emission B Limit Class B De- Height Polariza- f (MHz) (dB.mu.V/m) (dB.mu.V/m) Limit grees (m) tion ______________________________________ 196.608 35.8 30.0 5.8 90 1.5 Vertical 196.600 33.5 30.0 3.5 90 2.0 Horizontal 245.760 42.3 37.0 5.3 0 2.0 Vertical 245.75 37.9 37.0 0.9 90 2.5 Horizontal 540.660 30.0 37.0 -7.0 90 1.5 Vertical 540.660 39.5 37.0 2.5 90 1.0 Horizontal ______________________________________
As can be seen, the performance of the 6-to-4 cable does not meet the specifications provided by the FCC. Thus, current 6-to-4, and possibly 4-to-4 and 6-to-6 1394 cables can be prone to emit overly high amounts of electromagnetic radiation during the transmission of high-speed serial data.
Some prior art cables used for high-speed serial data transmission have failed to meet the standards for electromagnetic transmission set by the FCC. Thus, what is desired are improved methods and cables for the efficient transmission of high-speed serial data while minimizing electromagnetic emissions.